Candida albicans is an opportunistic fungus which generally exists in the oral cavity, skin, vagina and intestinal organs, and is also one of the causes of superinfection. 1) The antifungal drugs described below are applied to treat superficial and deep-seated infections. The arylamine antimycotic terbinafine inhibits squalene epoxidase in fungi. 2) Terbinafine induces the accumulation of squalene and interrupts the synthesis of ergosterol. Neticonazole is an antifungal drug of azole type that

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... nhibits ergosterol synthesis. 3) The polyene antimycotic amphotericin B binds to fungal membranes and obstructs the function of the membranes. 4) Ciclopirox olamine inhibits the membrane transfer system by interrupting Na ϩ K ϩ ATPase. 5) C. albicans is a dimorphic fungus which transforms from yeast to a hyphal form depending on the growth conditions. 6) AOX1, an alternative oxidase in C. albicans, seems to be related to the respiration system. 7) C. albicans has RAS1 protein, which activates factors involved in hyphal formation signaling, such as EFG1 and CPH1. 8) There may be differences in the metabolic and respiratory systems between yeast and hyphae cells, but the sensitivity of C. albicans to antifungal drugs has not been clarified. Examination of the sensitivity is important to do a suitable treatment of Candida infection. In this study, we clarified the sensitivity of C. albicans to antifungal drugs under various culture conditions and the effects of the drugs on the growth form of C. albicans. MATERIALS AND METHODS Fungus C. albicans NIH A207, TIMM1768, JCM2076 and TIMM2640 were cultured at 27°C for 24 h in Sabouraud's medium. Reagents Ciclopirox olamine (Azwell), siccanin (Sankyo), amphotericin B (Bristol-Myers Squibb), neticonazole (SSP) and terbinafine (Novartis Pharma) were used in this study. Sensitivity of C. albicans to Antifungal Drugs C. albicans cells (1ϫ10 5 cells/ml in RPMI1640 medium) were mixed with antifungal drugs and incubated at 37°C for 24 h under 5% CO 2 . After the incubation, the amount of C. albicans was measured as the optical density at 620 nm. The growth form of C. albicans cells was assessed by photo-The sensitivity of Candida albicans to antifungal drugs when cultured under aerobic and anaerobic conditions was measured. Ciclopirox olamine and siccanin were more effective under aerobic than under anaerobic conditions. Terbinafine, neticonazole and amphotericin B showed the same antifungal activity under both aerobic and anaerobic conditions. None of these antifungal activities were affected by the pH conditions. Terbinafine inhibited the elongation of hyphae, while neticonazole and amphotericin B induced proliferation of the yeast form. The expression of RAS1, EFG1 and CPH1 mRNAs was inhibited by these drugs. These results suggested that the inhibition of hyphal formation might be caused by disruption of the RAS1-signal pathway.